The communications system normally uses two channels, eg radio channels of two distinct frequencies. One channel is used by the node to send to all the outstations and the second channel is shared by all the outstations to send to the node. The system operates on a time sharing basis. Typical systems can provide service for 2 to 2000 outstations, eg 40 to 1000 outstations, using a frame period of 20 .mu.s to 200 ms, e.g., 100 .mu.s to 10 ms although not all outstations may have simultaneous access. Burst lengths (or slot lengths) are usually 1 .mu.s to 1 ms, preferably 10 .mu.s to 200 .mu.s.
The system requires clocks for timing the data. A transmitter utilizes a clock to control the rate of transmission; at the receiving end the receiver utilizes a clock to assist in data regeneration and it is important that the clocks be suitably synchronized. However, the system as a whole is scattered over substantial distances, e.g., a close outstation might be only 2 km from the node whereas a distant outstation might be 30 km from the node. Thus the transit (go and return) time for signals can range from 9 .mu.s to 90 .mu.s. Substantial difficulties result from this substantial variation in time of transit.